Since the last decade of the twentieth century, there has been increasing interest in building cameras that utilize the shortest wavelengths of the radio frequency (RF) portion of the electromagnetic spectrum that still penetrate materials. The range of interest typically starts at the “W band” (about 75 GHz to about 110 GHz) and extends into higher terahertz frequencies. The general concept is to perform imaging to detect and recognize objects of interest concealed behind cover materials such as clothing. This is passive imaging in that natural radiation from a scene is used to perceive differences in the apparent temperatures of objects.